1. Field of the Invention
The present invention relates to a method of reinforcing a molded body of ceramic by adding ceramic fibers and a reinforced molded body of ceramic which is produced by such a method, and more particularly to a method of reinforcing a molded body of ceramic by adding ceramic fibers which will not adversely affect the firing characteristics of the molded body of ceramic and a reinforced molded body which is produced by such a method.
2. Description of the Prior Art
Sintered bodies of ceramic are generally produced by molding a powder of ceramic into a molded body (green body) of desired shape and then firing the molded body. Since ceramic materials are difficult to machine once fired, they are molded into desired shapes before firing.
There have been known various processes for producing a molded body of ceramic. Among the presently available processes are a process of pressing a powder of ceramic with a die assembly, a process of injection-molding a ceramic material, a slip-casting process for casting a ceramic slurry or slip (or a kneaded mixture of resin or the like) into a plaster mold, and a cold isostatic pressing process of compacting a powder of ceramic with a rubber press. For producing molded bodies which are relatively thin or complex in shape, it is preferable to employ a slip-casting process, an injection-molding process, or a process using a doctor blade.
According to the slip-casting process, a ceramic material is ground into a powder, the powder is dispersed in water, producing a slurry or slip. The slurry is then poured into a plaster mold, and the water of the slurry is absorbed into and passed through the plaster mold, thereby forming a molded body (green body) of ceramic powder.
Ceramics of silicon nitride (Si.sub.3 N.sub.4) are finding wide use as substitutes for metallic materials because they are highly resistant to heat, highly mechanically strong, highly hard, and relatively lightweight. Since ceramics of silicon nitride are generally difficult to sinter, it has been customary to add a sintering additive of yttrium oxide (Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3), or the like and powders of other additives to a powder of silicon nitride, produce a molded body from a slurry of such a mixture according to the slip-casting process, and then fire the molded body.
However, a molded body of ceramic which is produced according to the slip-casting process does not have a sufficiently large mechanical strength and hence tends to break because the bonding strength between the particles of the ceramic powder is not necessarily large even if a binder is added to the ceramic powder. The molded body of ceramic is liable to shrink and crack due to dehydration and drying thereof in the plaster mold. For these reasons, it has been difficult to manufacture thin or complex molded bodies of ceramic without the danger of breaking or cracking.
The inventors have proposed a process of generating fibers of silicon (Si), nitrogen (N), carbon (C), and oxygen (O) by thermally decomposing polysilazane, and adding the generated fibers to a ceramic material to increase the mechanical strength of a molded body of ceramic, as disclosed in Japanese laid-open patent publication No. 4-238874. It has been found, however, that while the molded body of ceramic is being fired, some of the fibers are lost on account of different fiber compositions, leaving pores in the fired body. If a large amount of such fibers is added to a ceramic material, then the fired body of ceramic will contain many pores, resulting in a reduction in the mechanical strength of the fired body.